WANT to learn a musical instrument, but can’t find the time to practise? A device now under development can take control of your hand and teach you how to play a tune. No spirits of dead musicians are involved.

PossessedHand, being developed jointly by the University of Tokyo, Japan, and Sony Computer Science Laboratories, also in Tokyo, electrically stimulates the muscles in the forearm that move your fingers. A belt worn around that part of the subject’s arm contains 28 electrode pads, which flex the joints between the three bones of each finger and the two bones of the thumb, and provide two wrist movements. Users were able to sense the movement of their hands that this produced, even with their eyes closed. “The user’s fingers are controlled without the user’s mind,” explains Emi Tamaki of the University of Tokyo, who led the research.

Devices that stimulate people’s fingers have been made before, but they used electrodes embedded in the skin, which are invasive, or glove-like devices that make it hard to manipulate an object. Tamaki claims that her device is far more comfortable. “The electric stimulations are similar to low-frequency massage stimulations that are commonly used,” she says.

Having successfully hijacked a hand, the researchers tried to teach it how to play the koto, a traditional Japanese stringed instrument. Koto players wear different picks on three fingers, but pluck the strings with all five fingertips, so each finger produces a distinctive sound. A koto score tells players which fingers should be moved and when, and from this Tamaki and her team were able to generate instructions telling their device how and when to stimulate the wearer’s muscles.

PossessedHand does not generate enough force to pluck the koto strings, but it could help novice players by teaching them the correct finger movements. Tamaki and her team found that two beginner players made a total of four timing errors when using PossessedHand, compared with 13 when playing unassisted. After prompting from the device, the players also made one less mistake about which finger to use.

Perhaps not surprisingly, the players found it unsettling to have the device move their hand by itself. “I felt like my body was hacked,” said one. Tamaki is confident that people will get used to the idea once they see how useful it can be: “We believe convenient technology will overcome a feeling of fear.”

As well as helping would-be musicians, PossessedHand could be used to rehabilitate people who have suffered a stroke or other injury that impairs muscle control. Therapists already use electrical muscle stimulation to help these people, but existing non-invasive devices can only achieve crude movements such as contracting the entire arm.

Henrik Gollee, who researches rehabilitation devices at the University of Glasgow, UK, says PossessedHand could help patients train a wider range of movements. “I was surprised by the level of fine movement they can actually achieve,” he says.

Simon Holland, director of the Music Computing Lab at the Open University in Milton Keynes, UK, points out that there is a big difference between learning to play one song and being a competent musician. “You might learn a fingering and be able to reproduce that performance, without necessarily being able to perform simple variants,” he says.